1. Field of the Invention
The present invention relates to a tool to be fitted to a spindle included in a machine tool, such as a machining center and, more particularly, to a two-surface constrainable tool having a bottom grip taper shank (BT shank) with a conical taper shank and a flange to be constrained.
2. Description of the Related Art
BT shanks are used widely as shanks for standard tools for machining centers. The BT shank is mounted on a spindle with its taper shank fitted closely in the conical taper bore of the spindle. The recent progress of machining centers which achieve high performance in spindle speed and machining efficiency has brought about new problems in BT shanks. For example, a front end portion of the spindle surrounding the conical taper bore is caused to expand because of its high speed rotation and the tool holding firmness of the spindle is reduced. Consequently, the tool becomes unstable and chatters, and the positional accuracy of the cutting edge of the tool is deteriorated.
A new tool which has a BT shank with a taper part and a flange has been developed to solve such a problem. The taper part of the BT shank is fitted closely in the conical taper hole of a spindle and the flange is brought into contact with the end surface of the spindle to hold the tool securely on the spindle in a state of two-surface constraint. Known two-surface constrainable tools are shown in FIGS. 6 and 7.
FIG. 6 shows a two-surface constrainable tool known as a collar adjustment type. In FIG. 6 indicated at 30 is a spindle and at 32 is a BT shank. A taper part 33 of the BT shank 32 is fitted in a conical taper bore 31 formed in the spindle 30. A collar 35 is inserted into a clearance between the end surface of the spindle 30 and the end surface of a flange 34. According to the collar adjustment type, the taper part 33 of the tool is constrained by the inner surface of the conical taper bore 31, and the flange 34 of the same is constrained by the collar 35 in close contact with the end surface of the spindle 30.
FIG. 7 shows a two-surface constrainable tool known as of a century type. This tool has a BT shank 32 having a short taper part 36 and a flange 34. A disk spring 37 is placed in a clearance between the flange 34 and the end surface of a spindle 30 to facilitate pushing the short taper part 36 of the BT shank 32 into a taper bore formed in the spindle 30 and to enhance cutting vibration damping effect.
As to the BT shank 32 of the collar adjustment type retained in the two-surface constrained state, the collar 35 subjected to the centrifugal force is caused to loosen and the stiffness of contact between the collar 35 and the end surface of the spindle 30 decreases when the spindle 30 rotates at a high rotating speed.
On the other hand, the taper part 36 of the tool of the century type does not have a enough contact area to keep reliable contact with the surface of taper bore 31, and the tool is liable to play axially. There is an opinion that the tool of a century type is unsuitable for high-speed heavy machining.